Conformational Analysis of Guaianolide-Type Sesquiterpene Lactones by Low-Temperature NMR Spectroscopy and Semiempirical Calculations
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Conformational analysis of 9α-acetoxycumambrine A 1 and 8-O-isobutiryl-9α-acetoxycumambrine B 2 was carried out by low-temperature NMR studies. Results suggested that lactones 1 and 2 are mixtures of two distinctive conformers, I and II. Based on low-temperature 1H NMR spectra, in four solvents, the thermodynamic parameters of I ⇌ II exchange process were assessed. Energy of activation of I → II reaction was obtained by dynamic NMR simulations for both compounds. Results revealed that conformational exchange of lactones 1 and 2 occurs due to “chair ⇌ twisted chair” interconversion of a heptane ring. The same PM3 semiempirical method was applied for geometry optimization of lactones 1 and 2, as well as of 9α-hydroxycumambrine A 3, 9α-acetoxycumambrine B 4, and cumambrine B 5.
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